1. Field of the Invention
The invention is directed to a brush seal.
2. Description of the Prior Art
Such brush seals are utilized, for example, in gas turbines in order to seal the interspace between a housing and a rotor rotating at a comparatively high speed against leakage. In general, this ensues in the presence of hot, pressurized gas. Since no satisfactory sealing effect can be achieved under these conditions with simple rubber ring seals, numerous, different brush seals have been developed. Given these seals, the ends of the bristles that are combined to form a dense bristle packet project beyond the inside edge of an inside bore of the supporting plate and thus seal an interspace--to be kept optimally small--between the inside edge of the supporting plate and the rotor surface. This interspace is always present, since the dimensions of the seal, of the housing and of the rotor can change due to thermal stresses expansions or the like, and a minimum play between the supporting plate and the rotor must not be downwardly transgressed for safety reasons.
When a large differential pressure is adjacent the seal, then the supporting plate arranged at the low-pressure side supports the bristles to prevent sagging toward the low-pressure side in a direction proceeding parallel to the rotor axis. A sagging of the bristles is opposed because this reduces their length and, thus, opens the interspace between the inside edge of the supporting plate and the rotor surface, which leads to an increase of leakage. A sagging of the projecting bristle ends, however, cannot be avoided.
What is meant hereinabove and hereafter by an axial direction is a direction proceeding parallel to the longitudinal axis of the rotor, and what is meant by a radial direction or plane is a direction proceeding or plane extending perpendicular to the longitudinal axis of the rotor.
International Patent Document No. WO-A-86/05252 discloses a brush seal with a multitude of bristles in a holder for sealing two spaces differing in pressure, wherein the bristles are directed at an angle .beta. of 5.degree. through 45.degree. by setting means for improving the sealing effect toward the side of the higher pressure. When a differential pressure is adjacent, the bristles are pressed against the sealing surface from the side of higher pressure, which, on the one hand, leads to a dependable sealing effect and, on the other hand, however, leads to considerable wear of the bristles.
German Patent Document No. DE 3907 614 A1 discloses a brush seal wherein the bristles are clamped in a slotted tube that is clamped between two support rings joined to form a housing, wherein the inside edge of the support ring at the high-pressure side includes a greater spacing from the surface of a rotor than the inside edge of the support ring at the low-pressure side. A special design of the support ring at the low-pressure side, which is adapted to the bristles is not disclosed, nor is a setting of the bristles.
European Patent Application No. 0 778 431 A1 discloses a brush seal, wherein a support element arranged between the bristles--which are not set relative to the radial direction--and the low-pressure side is fashioned such that a chamber is present between the bristles and the support element and this chamber is supplied with fluid, whose pressure is higher than that at the low-pressure side, in order to increase the restoring force.
European Patent Application No. 0 617 216 A1 discloses a brush seal means with a support plate comprising a recess, the bristles being placed against the free end section thereof after being charged with pressure.
Given a brush seal disclosed by European Patent Application No. 0 453 315, the entire supporting plate serves as supporting surface against which the bristles--except for their projecting ends--lie over their entire, remaining length. For improving the sealing effect, the inside edge of an inside bore of the supporting plate is provided with an erodible material layer in order to lengthen the supporting surface and support the bristles close to their tips and to simultaneously reduce the interspace between the supporting plate and the rotor.
What proves disadvantageous given this seal, however, particularly given a differential pressure adjacent thereto, is that the bristles can deflect radially outward given an excursion of the rotor that, for example, can occur due to thermal stresses or expansions, but remain in this position even after return of the rotor into the central position as a consequence of aerodynamic effects. As a result of the lateral deflection of the bristles, the interspace between the supporting plate and the rotor is opened, so that the leakage increases. It is known that the bristles thereby adhere to the supporting plate and move back into their original position only after being completely relieved, i.e. a differential pressure of approximately zero, to close or seal the interspaces.